It has been proposed heretofore to provide transmissions of the stepped type which are capable of shifting under load, these transmissions generally comprising a housing having an input shaft and an output shaft and gearing, within the housing, in association with respective clutches which, upon selective actuation, establish respective transmission ratios between the input side of the transmission and the output shaft. Each of the clutches is associated with a respective step and by engaging one of the clutches while disengaging all others, the required transmission ratio can be obtained while the output shaft remains under load and the input is continuously applied.
It is also known, in association with such a stepped gear transmission capable of being shifted under load, to provide at the input side a hydrodynamic unit which generally consists of a hydrodynamic clutch or coupling and a torque converter. The torque converter is driven by, for example, an engine of the vehicle and is in turn connected to the input side or shaft of the gear transmission.
At the input side of the gear transmission, in addition, there can be provided a hydrodynamic retarder and the hydrodynamic elements of the arrangement can be supplied with hydraulic fluid or the hydraulic fluid flow paths can be established via one or more control valves.
Transmissions of the aforedescribed type are generally characterized by relatively high displacement rates of hydraulic fluid for the hydrodynamic unit (torque converter) and the retarder.
In a conventional transmission shiftable under load of this type, there is provided an intermediate wall or housing cover as the housing closure between a first housing section (gear transmission chamber) which must be a lubricated space and the second housing section (converter chamber) which generally remains dry, i.e. does not receive free lubricant or oil.
This housing closure portion contains, in the conventional system and usually axially offset from the input shaft, a hydraulic-fluid feed pump while the control valves are disposed in a control housing beneath the gear trains provided in the first housing section.
As a result of this configuration, long flow paths are provided from the pump through the corresponding control valves to the torque converter and retarder. These long flow paths are characterized by large pressure drops and like flow losses so that the filling of the retarder may require an unsatisfactorily long time. With this construction, moreover, it is practically impossible to avoid a large number of locations at which transfer of fluid through the fluid passages can occur between the individual housing sections. This is especially disadvantageous when the main pressure valve is mounted in the housing-closure cover.
It is also known, see for example published German application (Auslegeschrift) DT-AS No. 1 134 903, to provide a hydrodynamic converter which is separated by a partition from a retarder connected thereto, the partition containing the drive for a fluid pressure pump. The stator of the retarder is directly connected to this partition.
Since in this case the control valves are also separated from the torque converter and the retarder in a separate housing, the same disadvantage arises as in the arrangement described above.